The present invention relates to a beam compression process for compressing a beam width of an antenna pattern of antennas.
Generally, a beam width is one of indices representing quality of antenna patterns of receiving antennas and so forth. The smaller the beam width, the higher is performance of the antenna pattern. However, the beam width and the size (length) of an antenna are inversely proportional to each other. Thus, a reduction in the beam width increases the antenna size, while a reduction in the antenna size increases the beam width.
In an attempt to double power of discriminating an object, i.e., resolution, in a radar antenna, for example, the beam width must be halved, which results in doubling of the antenna size. The doubled antenna size raises various drawbacks such as an increase in not only the area occupied by the antenna, but also weight of the antenna and dimensions of an antenna support structure. Conversely, if the antenna size is halved, the beam width is doubled and the discriminating power deteriorates down to a half level.
It is well known that the beam width and the antenna size are contradictory to each other as mentioned above. Since actual antennas are subjected to limitations in the area occupied by the antenna and other factors in most cases, a point of compromise is found in practical use at some extent of the beam width under such limitations.
For the purpose of improving the above problem, there has been conventionally known a beam compressing process using the principle of a multiplicative array that the beam width is reduced by multiplying received signals of a plurality of antennas by each other. FIG. 1 is a diagram showing an antenna arrangement for carrying out such beam compression. Denoted by reference numeral 101 is a main antenna constituted by, for example, an array antenna comprising a plurality of radiation elements arrayed into the rectilinear form with equal intervals, and 102 is a sub antenna. The sub antenna 102 is arranged at a position spaced from the main antenna 101 in the X direction, i.e., the direction where a beam width is to be compressed. 103 is a multiplying circuit for multiplying a received signal of the main antenna 101 by a received signal of the sub antenna 102. In the antenna device thus arranged, the signals received by the antennas 101, 102 are input in phase to the multiplying circuit 103 and subjected to a multiplication process. As a result, a directional characteristic of the main antenna and a directional characteristic of the sub antenna are multiplied to give a synthetic directional characteristic with the beam width compressed therein.
However, the above-explained conventional beam compression process for an antenna pattern has the problem that because the angle corresponding to the first zero point of the sub antenna pattern is about 1/2 of the angle corresponding to the first zero point of the main antenna pattern, the beam width of the main antenna is only compressed to about a half and cannot be compressed to less than a half.